IEEE (Institute of Electrical and Electronic Engineers) 802.16 is an emerging suite of air interface standards for combined fixed, portable and Mobile Broadband Wireless Access (MBWA). Initially conceived as a radio standard to enable cost-effective last-mile broadband connectivity to those not served by wired broadband such as cable or DSL, the specifications are evolving to target a broader market opportunity for mobile, high-speed broadband applications. The IEEE 802.16 architecture not only addresses the traditional “last mile” problem, but also supports nomadic and mobile clients on the go. The MBWA architecture is being standardized by the IEEE 802.16 Working Group and the Worldwide Interoperability for Microwave Access (WiMAX) forum. For convenience, the terms 802.16 and WiMAX are used interchangeably throughout this specification to refer to the IEEE 802.16 suite of air interface standards.
FIG. 1 shows a simplified broadband wireless network with point-to-multipoint (PMP) architecture for operation at both licensed and licensed-exempt frequency bands typically below 11 GHz. Other types of architectures (not shown) such as mesh broadband wireless networks are permissible. A backbone IP (Internet Protocol) network 100 is connected to a broadband wireless network using radio access nodes (RANs) 102A and 102B. Each RAN is connected via a wired link such as an optical fiber (depicted as optical fiber links 103A, 103B and 103C) or point-to-point wireless link (not shown) to one or more radio cells (depicted between RAN 102A or 102B to radio cells 104A, 104B, and 104C). At the hub of a radio cell is a respective Base station (BS) 106A, 106B, and 106C. A Base Station system includes an advanced antenna system (AAS), which is typically located on top of a radio tower and is used to transmit high-speed data to multiple subscriber stations (SSs) 108 and receive data from the subscriber stations via unidirectional wireless links 110 (each SS uplink transmission is independent on the others). More particularly, each SS 108 can access network 100 (via an appropriate BS) using the PHY+MAC (Physical+Media Access Control) layer features defined by the IEEE P802.16 air-interface standard. An SS may correspond to a fixed subscriber location (e.g., in a home or office), or may correspond to a mobile subscriber who might access the broadband wireless network via a mobile device such as a personal digital assistant (PDA), laptop computer, etc. A fixed SS typically uses directional antenna while a mobile or portable SS usually uses an omni-directional antenna.
Transmission of data bursts from network 100 to an SS 108 proceeds in the following manner. The data bursts such as IP packets or Ethernet frames are encapsulated in IEEE 802.16-2004 data frame format and forwarded from an appropriate RAN to an appropriate BS within a given cell. The BS then transmits non-line of sight (NLOS) data to each SS 108 using a unidirectional wireless link 110, which is referred to as a “downlink.” Transmission of data from an SS 108 to network 100 proceeds in the reverse direction. In this case, the encapsulated data is transmitted from an SS to an appropriate BS using a unidirectional wireless link referred to as an “uplink.” The data packets are then forwarded to an appropriate RAN, converted to IP Packets or Ethernet frames, and transmitted henceforth to a destination node in network 100. Data bursts can be transmitted using either Frequency-Division-Duplexing (FDD) or Time-Division-Duplexing (TDD) schemes. In the TDD scheme, both the uplink and downlink share the same RF channel, but do not transmit simultaneously, and in the FDD scheme, the uplink and downlink operate on different RF channels, sometimes simultaneously.
Multiple BSs are configured to form a cellular-like wireless network. A network that utilizes a shared medium requires a mechanism to efficiently share it. Within each cell, the wireless network architecture is a two-way PMP, which is a good example of a shared medium; here the medium is the space (air) through which the radio waves propagate. The downlink, from the base station (BS) to an SS, operates on a PMP basis. Provisions within the IEEE 802.16-2004 standard include a central BS with AAS within each cell. Such an AAS includes a sectorized antenna that is capable of handling multiple independent sectors simultaneously. Under this type of configuration, the operations of base stations described below may be implemented for each of the independent sectors, such that multiple co-located base stations with multiple sector antennas sharing a common controller may be employed in the network. Within a given frequency channel and antenna sector, all stations receive the same transmission, or parts thereof.
In the other direction, the subscriber stations share the uplink to the BS on a demand basis. Depending on the class of service utilized, the SS may be issued continuing rights to transmit, or the right to transmit may be granted by the BS after receipt of a request from an SS. In addition to individually-addressed messages, messages may also be sent on multicast connections (control messages and video distribution are examples of multicast applications) as well as broadcast to all stations. Within each sector, users adhere to a transmission protocol that controls contention between users and enables the service to be tailored to the delay and bandwidth requirements of each user application.